1,195 research outputs found
Finite temperature Z(N) phase transition with Kaluza-Klein gauge fields
If SU(N) gauge fields live in a world with a circular extra dimension,
coupling there only to adjointly charged matter, the system possesses a global
Z(N) symmetry. If the radius is small enough such that dimensional reduction
takes place, this symmetry is spontaneously broken. It turns out that its fate
at high temperatures is not easily decided with straightforward perturbation
theory. Utilising non-perturbative lattice simulations, we demonstrate here
that the symmetry does get restored at a certain temperature T_c, both for a
3+1 and a 4+1 dimensional world (the latter with a finite cutoff). To avoid a
cosmological domain wall problem, such models would thus be allowed only if the
reheating temperature after inflation is below T_c. We also comment on the
robustness of this phenomenon with respect to small modifications of the model.Comment: 18 pages. Revised version, to appear in Nucl.Phys.
Meson Correlation Function and Screening Mass in Thermal QCD
Analytical results for the spatial dependence of the correlation functions
for all meson excitations in perturbative Quantum Chromodynamics, the lowest
order, are calculated. The meson screening mass is obtained as a large distance
limit of the correlation function. Our analysis leads to a better understanding
of the excitations of Quark Gluon Plasma at sufficiently large temperatures and
may be of relevance for future numerical calculations with fully interacting
Quantum Chromodynamics.Comment: 11 page
Mass and chemical asymmetry in QCD matter
We consider two-flavor asymmetric QCD combined with a low-energy effective
model inspired by chiral perturbation theory and lattice data to investigate
the effects of masses, isospin and baryon number on the pressure and the
deconfinement phase transition. Remarkable agreement with lattice results is
found for the critical temperature behavior. Further analyses of the cold,
dense case and the influence of quark mass asymmetry are also presented.Comment: 4 pages, 4 figures; to appear in the Proceedings of Strong and
Electroweak Matter 2008 (SEWM08), August 26-29, Amsterdam, The Netherland
A Brane model with two asymptotic regions
Some brane models rely on a generalization of the Melvin magnetic universe
including a complex scalar field among the sources. We argue that the geometric
interpretation of Kip.S.Thorne of this geometry restricts the kind of potential
a complex scalar field can display to keep the same asymptotic behavior. While
a finite energy is not obtained for a Mexican hat potential in this
interpretation, this is the case for a potential displaying a broken phase and
an unbroken one. We use for technical simplicity and illustrative purposes an
ad hoc potential which however shares some features with those obtained in some
supergravity models. We construct a sixth dimensional cylindrically symmetric
solution which has two asymptotic regions: the Melvin-like metric on one side
and a flat space displaying a conical singularity on the other. The causal
structure of the configuration is discussed. Unfortunately, gravity is not
localized on the brane.Comment: 9 pages revtex, 4 figures,version to appear in PR
A simple way to generate high order vacuum graphs
We describe an efficient practical procedure for enumerating and regrouping
vacuum Feynman graphs of a given order in perturbation theory. The method is
based on a combination of Schwinger-Dyson equations and the
two-particle-irreducible ("skeleton") expansion. The regrouping leads to
skeletons containing only free propagators, together with "ring diagrams"
containing all the self-energy insertions. As a consequence, relatively few
diagrams need to be drawn and integrations carried out at any single stage of
the computation and, in low dimensions, overlapping ultraviolet/infrared
subdivergences can be cleanly isolated. As an illustration we enumerate the
graphs contributing to the 4-loop free energy in QCD, explicitly in a continuum
and more compactly in a lattice regularization.Comment: 19 pages. Reference added. To appear in Phys.Rev.
Masses and Phase Structure in the Ginzburg-Landau Model
We study numerically the phase structure of the Ginzburg-Landau model, with
particular emphasis on mass measurements. There is no local gauge invariant
order parameter, but we find that there is a phase transition characterized by
a vanishing photon mass. For type I superconductors the transition is of 1st
order. For type II 1st order is excluded by susceptibility analysis, but the
photon correlation length suggests 2nd order critical behaviour with \nu ~ 1/2.
The scalar mass, in contrast, does not show clear critical behaviour in the
type II regime for V \to \infty, contrary to the conventional picture.Comment: 16 pages, 6 figures. More data gathered, allowing more definite
conclusion
The monopole mass in the three-dimensional Georgi-Glashow model
We study the three-dimensional Georgi-Glashow model to demonstrate how
magnetic monopoles can be studied fully non-perturbatively in lattice Monte
Carlo simulations, without any assumptions about the smoothness of the field
configurations. We examine the apparent contradiction between the conjectured
analytic connection of the `broken' and `symmetric' phases, and the
interpretation of the mass (i.e., the free energy) of the fully quantised 't
Hooft-Polyakov monopole as an order parameter to distinguish the phases. We use
Monte Carlo simulations to measure the monopole free energy and its first
derivative with respect to the scalar mass. On small volumes we compare this to
semi-classical predictions for the monopole. On large volumes we show that the
free energy is screened to zero, signalling the formation of a confining
monopole condensate. This screening does not allow the monopole mass to be
interpreted as an order parameter, resolving the paradox.Comment: 12 pages, 7 figures, uses revtex. Minor changes made to the text to
match with the published version at
http://link.aps.org/abstract/PRD/v65/e12500
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